Remove the feature flag for forced_app_standby

The forced-app-standby or background-restriction feature is now a stable
feature that has been in multiple releases. The code for toggling it on
or off is no longer required.

Test: atest CtsAlarmManagerTestCases:BackgroundRestrictedAlarmsTest
Test: atest FrameworksMockingServicesTests:AppStateTrackerTest
Test: atest CtsJobSchedulerTestCases:JobThrottlingTest
Test: Manually test that background restriction UI works as expected.

Bug: 265350615
Change-Id: I109dd97f9918e97ab2406b99ce15bd991071b7c7